It is well known in the art to utilize the Doppler effect to monitor the velocity of a material, such as grain, moving on a conveyor or flowing through a pipe or tube. An article entitled Microwave Doppler-Effect Flow Meter, in the publication IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. IECI-22, May 1975, pg. 224-228, describes Doppler effect flow monitors utilizing horn antennas arranged in a monostatic or bistatic configuration, that is arranged with a single transmit/receive antenna disposed on one side of the material under test, or a transmit and a receive antenna disposed on opposite sides of the material.
FIG. 7 illustrates a Doppler effect flow monitor in the monostatic configuration as described in the publication. A transmit/receive horn antenna 10 is disposed so that the axis of the radiation pattern emitted by the antenna lies at an angle .theta. (optimally 45.degree.) relative to the direction of grain flow indicated by arrow 12. The grain is contained by a pipe 14 having a pipe section 16 made of a material transparent to the radiated frequency. The section 16 is covered by a layer 18 of material which absorbs RF at the frequency emitted by the antenna. The layer 18 is provided with a window 20 to permit entry of radiation into the pipe and passage of reflected RF from the grain back to the horn antenna. The described purpose of layer 18 is to absorb and minimize extraneous reflections from surfaces other than the grain particles.
Antenna 10 radiates or transmits a continuous wave microwave signal of fixed frequency through window 20 and the wall of RF transparent pipe section 6 so as to intercept individual grain particles moving through the pipe. The transmitted signal is reflected back toward the antenna 10 at a second frequency which depends on the velocity of the grain. The frequency difference between the frequency of the transmitted signal f and the frequency of the reflected signal is the Doppler frequency f.sub.d. By counting cycles of the Doppler signal over an interval of time, the grain velocity can be calculated according to the equation EQU v=(c/2f cos .theta.)f.sub.d (1)
where c is the speed of light in m/s, f is the frequency of the transmitted signal in Hz, f.sub.d is the frequency of the Doppler signal in Hz and .theta. is the antenna viewing angle, that is, the angle between the axis of the main lobe of the radiation pattern and the axis of the pipe section through which the grain is moving.
In actual practice, the reflected signal comprises a spectrum of Doppler frequencies resulting from interception of the transmitted signal at different angles by individual grain particles moving at different speeds. As illustrated in FIG. 7, the angles of interception extend over a range from .theta..sub.L, larger than the angle .theta., to the angle .theta..sub.s smaller than the angle .theta.. Although the off-axis interceptions affect the precision of the measurement, the average Doppler frequency f.sub.d is proportional to the average velocity v so that the velocity may be calculated with a fair degree of precision according to the formula EQU v=(c/2f cos .theta.)f.sub.d (2)
In FIG. 7, a transceiver 22 energizes antenna 10 to transmit the continuous wave microwave signal, and detects the signal reflected back to the antenna from the grain particles. The reflected and detected signal which, as noted above, comprises a spectrum of frequencies, is processed in a signal processing unit 24. The time and complexity of the processing is dependent on the number of frequencies or width of the spectrum of frequencies of the reflected signal since the processing requires computation of a plurality of Doppler frequencies f.sub.d and an averaging of these frequencies to obtain f.sub.d before the average velocity v may be calculated.
Once the average velocity is calculated, processing unit 24 calculates the mass flow rate of the grain according to the formula EQU Q=ADv
where Q is the mass flow rate in kg/s, A is the cross-sectional area in m.sup.2 of the grain flow path within pipe 14, D is the bulk density of the grain in kg/m.sup.3, and v is the average velocity in m/s. The value of Q may then be displayed on an indicator 26.
The publication discussed above does not directly mention the problem of Doppler error introduced into the velocity measurement by side lobe responses of the transmitted radiation pattern when the antenna is placed in close proximity to the grain flow path. The authors do suggest that the optimum distance between the antenna aperture and the axis of the flow pipe be between W.sup.2 /.pi.2 and 2W.sup.2 /.pi.2 where W is the width of the antenna aperture which controls the radiation pattern of interest. In experiments described in the publication, the antenna was placed at a distance of 0.1 m from the wall of the flow pipe.
Generally speaking, Doppler errors due to side lobe responses have been reduced in Doppler velocity monitors by appropriate selection of the distance between the antenna and the flow path. The shortest distance where the side lobes are not a problem is a function of antenna gain and the wavelength of the transmitted signal. The conventional approach has been to place the antenna at the closest distance to the flow path where the side lobe components do not add to the Doppler frequency spectrum, and this may vary from several inches to many feet. This requirement, together with the physical size of horn antennas, makes such antennas unsuitable for use in practical applications where only limited space is available for the antenna.
U.S. Pat. No. 2,591,486 teaches the use of dielectric spacers to absorb side lobes of a horn antenna in a radio echo detection system. However, the use of spacers disposed as taught in this patent would not serve to reduce the Doppler spectrum resulting from main lobe responses in a Doppler velocity monitor.
High gain antennas comprising a phased array of antenna elements on a printed circuit antenna card are commercially available. These antennas are much smaller than horn antennas thus making them more suitable for use when space is a limiting factor.
The beam width of the main lobe radiated by a high gain antenna decreases as the gain increases. The narrow beam width makes the antennas desirable for use in flow monitors because it reduces the range of the angles of incidence of the beam on the material under test, thus reducing the width of the Doppler frequency spectrum. However, to realize the narrow beam width the antenna must be spaced a considerable distance from the material under test thus making the antenna less desirable for use where only limited space is available.